The present invention relates to an apparatus for producing fine metal balls having narrow particle size distribution and high sphericity.
There has recently been an increasing demand in extremely many fields for fine metal balls having narrow particle size distribution and high sphericity, such as solder balls used for the microsoldering of semiconductor devices, metal powder for producing sintered alloys by hot isostatic pressing, balls for extremely small ball bearings used for micro-machines, light-emitting particles sealed in metal halide lamps, powder used for pastes, creams or paints for screen printing or immersion coating and other coating machines.
For instance, in the case of solder balls, they are required to be as spherical as possible for use in the assembling of semiconductor devices. Widely used as a technique for mounting semiconductor devices with solder balls is BGA (ball grid array) called CSP (chip size package), MCM (multi-chip module), etc. To connect a semiconductor device to a substrate with a pad of a BGA carrier with a bump, it is necessary to arrange several hundreds of, or in many cases several thousands of, solder balls per an array on the carrier. Also, in semiconductor devices becoming smaller like LSI, VLSI and ULSI, there is an increasingly larger demand for making the solder balls finer, more spherical and narrower in a particle size distribution.
The production of fine metal balls is conventionally carried out by apparatuses using an atomizing method, a uniform droplet spray method, etc. Any of these apparatuses is constituted by a crucible for holding a metal melt and equipped at a bottom thereof with nozzles for ejecting the melt, and a solidification chamber connected to the bottom of the crucible. A cooling system in the solidification chamber may be a gas-cooling system or an in-oil cooling system. For instance, in a gas-cooling apparatus the crucible is pressurized while giving vibration at a constant frequency to the melt, such that the melt is ejected through the nozzles at a constant speed into the solidification chamber, in which the melt is turned to spherical melt droplets by its own surface tension while dropping in the solidification chamber. The spherical melt droplets are cooled by a gas in the solidification chamber to be solidified and deposited on the bottom of the solidification chamber. This gas-cooling system is also called a uniform droplet spray method, suitable for mass-producing fine solidified metal balls having uniform particle size and shape.
For instance, an apparatus disclosed by U.S. Pat. No. 5,266,098 for producing solder balls by a uniform droplet spray method comprises, as shown in FIG. 12, a crucible 3 having a plurality of orifices 2 at the bottom, a vibration rod 6 for vibrating a melt in the crucible 3, a disc 71 connected to thereto, a piezoelectric vibrator 4 connected to the vibration rod 6, a member 81 supporting the piezoelectric vibrator 4 and movable in a vibration direction, and a charging means 85 for giving electric charge to melt droplets dropping from the orifices 2. The melt 1 is ejected through a plurality of orifices 2 at the bottom of the crucible 3, turned to independent melt droplets by vibration given to the melt 1, and solidified.
To produce fine metal balls having a narrow particle size distribution by a uniform droplet-dropping apparatus, it is important to suppress the frequency variation of vibration given to the melt and the speed variation of the melt ejected through the orifices. As a method for giving vibration to the melt at a constant frequency, U.S. Pat. No. 5,266,098 describes a method using a piezoelectric element to give vibration to a melt from outside. Though it may be considered that the melt is ejected at a constant speed because it utilizes the accurate vibration of the piezoelectric element, fine metal balls were not necessarily produced stably by the apparatus of U.S. Pat. No. 5,266,098 shown in FIG. 12, because the apparatus stopped abruptly during a production process. In addition, it has been found that the fine metal balls produced by the apparatus of U.S. Pat. No. 5,266,098 have large variations in a particle size distribution and a sphericity distribution among production lots.
In the apparatus shown in FIG. 12, when the high-frequency vibration of the piezoelectric vibrator 4 is transmitted to the vibration rod 6 and the vibration disc 71 connected thereto, there arises a large concentration of stress in a vibration-transmitting portion from the piezoelectric vibrator 4 to the vibration rod 6. This concentration of stress makes the piezoelectric vibrator 4 unstable, which is considered a main reason of the stop of the apparatus. Also, a stress component in an undesirable direction acts on the vibration-transmitting portion, causing sliding in parts, etc. and thus wearing their contact portions. Heat generated by wear exerts adverse effects on the life of the piezoelectric vibrator 4, which is also considered as a reason for stopping the apparatus. It may further be considered that because the transmission of this vibration is a planar transmission with a constant cross section, slight tolerance, etc. of mechanical mounting portions generates difference in the transmission of vibration to the melt, resulting in unevenness in the quality of fine metal balls among production lots.
The speed of the melt ejected through the orifices 2 is determined by difference between the pressure of the melt 1 exerting on the vicinity of the orifices 2 in the crucible 3 and a gas pressure in a solidification chamber (not shown), etc. The pressure of the melt 1 exerting on the vicinity of the orifices 2 in the crucible 3 decreases in proportion to the amount of the melt in the crucible 3, which decreases as the melt 1 is ejected. On the other hand, the gas pressure in the solidification chamber increases in proportion to a temperature inside the solidification chamber, which is elevated by the quantity of heat and heat of solidification of the melt ejected. Thus, the difference in pressure between the crucible 3 and the solidification chamber decreases as the ejection of the melt into the solidification chamber proceeds, resulting in change in the cooling speed of the melt droplets 9 accordingly. It is thus considered that there arises unevenness in the solidification structure of the fine metal balls produced.
Accordingly, an object of the present invention is to provide an apparatus for stably producing fine metal balls having a narrow particle size distribution and a high sphericity by a uniform droplet spray method.
Another object of the present invention is to provide an apparatus for producing fine metal balls, wherein the ejection speed of the melt is easily kept constant.
A further object of the present invention is to provide an apparatus for producing fine metal balls having a homogeneous solidification structure, wherein a gas pressure in a solidification chamber can easily be controlled.
The apparatus for producing fine metal balls according to the present invention comprises a crucible for holding a metal melt and equipped with orifices for ejecting the metal melt; a vibration rod for giving vibration to the melt held in the crucible; a vibrator for giving vibration to the vibration rod; a means for transmitting the vibration of the vibrator to the vibration rod; and a chamber in which melt droplets ejected through the orifices are solidified while dropping, the vibration-transmitting means having one end in contact with the vibrator and the other end abutting a support member connected to the vibration rod; the vibration-transmitting means having a cross section decreasing as it nears the support member. The vibration-transmitting means preferably has a tip end portion substantially in a hemispherical or half-cylindrical shape.
In one embodiment of the present invention, the solidification chamber comprises a means for bounding the solidified fine metal balls a plurality of times so that the kinetic energy of the fine metal balls is attenuated by a plurality of steps. Specifically, the solidification chamber comprises a slanting surface member at a position at which the fine metal balls land, and an inner surface of the solidification chamber and the slanting surface member are covered with shock-absorbing means such as rubber, such that the fine metal balls are caused to bound a plurality of times between both shock-absorbing means.
In another embodiment of the present invention, the apparatus for producing fine metal balls comprises (a) a melt-ejecting means comprising a melt-ejecting crucible for holding a metal melt and equipped with orifices for ejecting the metal melt, a pressure-controlling means for controlling the pressure of the melt ejected through the orifices, and a vibration means for giving vibration to the melt; (b) a melt supplier comprising one or more melt supply crucibles for holding the melt, and one or more conduits connecting the melt supply crucibles to the melt-ejecting crucible; and (c) a solidification chamber in which melt droplets ejected through the orifices are solidified while dropping. The melt supply crucible is preferably equipped with a metal material-supplying means.
The vibration means in this embodiment comprises a vibration rod for giving vibration to the melt held in the melt-ejecting crucible, a vibrator for giving vibration to the vibration rod, and a means for transmitting the vibration of the vibrator to the vibration rod, the vibration-transmitting means having one end in contact with the vibrator and the other end abutting a support member connected to the vibration rod, and the vibration-transmitting means having a cross section decreasing as it nears the support member.
The melt supply crucible is preferably equipped with an elevating means for changing the height thereof. Each of the melt-ejecting crucible and the melt supply crucible is equipped with a gas-pressure controlling means for controlling a gas pressure applied to a surface of the melt held therein. Using an inert gas or a reducing gas for gas pressure control, the oxidation of the melt can be prevented.
The melt supply crucible can be equipped with a means for detecting the surface position of the meld held therein or a load cell for detecting the change of the weight of the melt, to determine the weight of the melt ejected from the melt-ejecting crucible.
In a further embodiment of the present invention, the apparatus for producing fine metal balls comprises a melt-ejecting means comprising a crucible for holding a metal melt and equipped with orifices for ejecting the metal melt, and a vibration means for giving vibration to the melt; a solidification chamber in which melt droplets ejected through the orifices are solidified while dropping; and a heat-exchanging means for controlling the temperature of an atmosphere inside the solidification chamber. The heat-exchanging means preferably comprises a heat-exchanging portion mounted onto an inner wall of the solidification chamber, a coolant circulating in the heat-exchanging portion, and a temperature controller for controlling the temperature of the coolant.
The vibration means in this embodiment comprises a vibration rod for giving vibration to the melt held in the crucible, a vibrator for giving vibration to the vibration rod, and a means for transmitting the vibration of the vibrator to the vibration rod, the vibration-transmitting means having one end in contact with the vibrator and the other end abutting a support member connected to the vibration rod, and the vibration-transmitting means having a cross section decreasing as it nears the support member.
The heat-exchanging means preferably comprises a sub-chamber connected to the solidification chamber such that a gas can be circulated therebetween, a heat-exchanging portion mounted in the sub-chamber, a coolant circulating in the heat-exchanging portion, and a temperature controller for controlling the temperature of the coolant. The temperature controller preferably controls the temperature of the coolant based on the gas temperature detected by a thermometer mounted in the solidification chamber.